WFPC 2 Dark Current vs . Time

Ongoing measurements of the dark current in the WFPC2 detectors indicate that the average level of dark current has been slowly increasing over the instrument's lifetime. We have analyzed more recent dark current data, now covering the period from September 1994 to January 2001. This analysis shows that after late-1998, the dark current is increasing more slowly than expected and could possibly be leveling off temporarily. The STIS CCD dark current shows a similar effect. This behavior may be linked to the solar cycle, where the cosmic ray rate is reduced at solar maximum, causing the median dark current to be lower. If this hypothesis is correct, we expect the dark current to eventually continue increasing at its previous rate.